The overall goal of this project is to understand how the turnover of phospholipid acyl groups is regulated in intact cells. Based on recent work carried out with rat hepatocytes, we are able to distinguish de novo synthesis from remodeling through deacylation-reacylation by following the incorporation of 18(O) from H2 18(O)-containing media into individual molecular species of phospholipids. This novel approach to quantifying lipid turnover will be supplemented with conventional radioisotope techniques utilizing [14C] glycerol. Under Aim 1 we will examine the hypothesis that, in hepatocytes, de novo phospholipid synthesis and remodeling are quantitatively similar and closely linked to each other and to intracellular phospholipic transport and degradation, that there exist specific precursor-product relationships among phospholipid molecular species involving exchanges of acyl groups at the sn-1 and sn-2 positions of glycerol, and that alterations in the fatty acid composition of membrane phospholipids occur primarily through the de novo pathway. Under Aim 2 we will determine whether or not oxidative stress selectively enhances turnover of polyunsaturated acyl groups in membrane phospholipids. We suspect that acyl turnover through de novo synthesis, remodeling, transport and complete deacylation is sufficiently rapid and complete to eliminate peroxidized fatty acids and to accomplish membrane "repair" through recycling of the components of complex lipids rather than through selective replacement of oxidatively modified acyl groups in situ. Under Aim 3 we will compare parameters of phospholipid acyl turnover between rat hepatocytes and mouse peritoneal macrophages. We expect to find similar relationships between de novo synthesis and remodeling of diacylglycerophospholipids and to obtain new information on the turnover of their alkylacyl and alkenylacyl analogs. Finally, we expect to obtain evidence on how stimulated acyl turnover and fatty acid liberation are superimposed on acyl turnover in testing cells.